Infrared Astronomy

  IRAS all sky infrared image

The infrared region lies between wavelengths of about 1 and 300 microns (a micron is one millionth of a meter). In the middle of the 19th Century, astronomers began to observe in the infrared and our speaker, Dr Chris Baddiley took us through some of the history of this fascinating subject.

The first person to investigate infrared emissions was the astronomer William Herschel back in the year 1800. He shone light through a prism which split up the white rays into different colours just like a rainbow. He placed thermometers into the different coloured beams and noted what temperature they read. As his measurements progressed towards the red end of the spectrum he saw that the temperature increased. To his amazement he saw that the greatest temperature was recorded when the thermometer was just beyond the visible red light beam where he was unable to see any light at all. He called these mysterious rays "calorific rays" and we know them today as infrared radiation; where the prefix 'infra' means below.

The first object to be scrutinized in the infrared was our Moon and it had its temperature taken by the 4th Earl of Rosse. It was estimated to be about 260 C which was only about 22 degrees too cool. An impressive result considering that he used a primitive device called a thermocouple, which is an electric circuit that is sensitive to temperature. The next object was the Sun only eight years later, when astronomers took advantage of the newly invented "bolometer" which was sensitive to differences in temperature of one hundred-thousandth of a degree Celsius.

Gradually the observations were edging further and further out from Earth and with each new technical advance so could the researchers see further and in greater detail. Like their fellow optical astronomers, infrared specialists began to build observatories at high altitude, launch balloons, rockets and orbiting spacecraft. Anything to escape the effects of water vapour and carbon dioxide in our atmosphere, that not only sucks up infrared light like a sponge but is already so hot that it masks any external infrared source.

One of the main advantages of observing in the infrared is that it is not absorbed by the vast dust clouds of our galaxy. We are able to see deep into our galaxy's core and watch stars swirl around the massive black hole that is thought to lurk at its very centre. These observations have helped to confirm the black hole's mass; at a whopping 3 to 4 million times that of our own Sun. One rather unconventional infrared observing platform is NASA's highly modified Boeing 747 which houses a 2.5 metre reflecting telescope. Known as SOFIA, it is the largest airborne observatory in the world. It will fly into the stratosphere, open the telescope cavity door, and point its telescope at the heavens three or four nights a week for at least twenty years beginning in the autumn of 2005.